The present invention relates to a hybrid automatic transmission suitable for a hybrid electric vehicle (HEV) mounting thereon a prime mover such as an internal combustion engine and at least one motor-generator, and specifically to the improvement of a hybrid automatic transmission having a differential device disposed between the prime mover and the motor-generator to steplessly vary a transmission gear ratio.
In recent years, there have been proposed and developed various hybrid electric vehicles equipped with hybrid automatic transmissions. One such hybrid automatic transmission has been disclosed in Japanese Patent Provisional Publication No. 2000-238555 (hereinafter is referred to as xe2x80x9cJP2000-238555xe2x80x9d). FIG. 9 shows a schematic structural drawing of the hybrid automatic transmission as disclosed in JP2000-238555. The hybrid transmission shown in FIG. 9 uses a simple planetary gearset 31 that is comprised of a sun gear 31s, a ring gear 31r, and a planet-pinion carrier 31c. Input torque from an internal combustion engine is transmitted via a transmission input shaft 32 to carrier 31c. On the one hand, a part of the input torque transmitted to carrier 31c is transmitted through sun gear 31s and a cylindrical hollow shaft (or a sun-gear shaft) 33 to a first motor-generator 34 serving as a generator. On the other hand, the remaining engine torque is transmitted through ring gear 31r, a sprocket 35, a chain belt 36, and a differential gear 39 to drive wheels 37, 37. The ring-gear shaft of ring gear 31r is connected to the rotor of a second motor-generator 38 serving as an electric motor.
Referring now to FIGS. 10 and 11, there are shown alignment charts representing the construction of the simple-planetary-gearset equipped hybrid automatic transmission system shown in FIG. 9. Simple planetary gearset 31 is a differential device having a three-element, two-degree-of-freedom. Thus, second motor-generator 38 (serving as the motor) is connected directly to ring gear 31r serving as an output element to which a drive train containing differential gear device 39 is connected. As can be seen from the alignment chart of FIG. 10, first motor-generator 34 (serving as the generator) is connected to sun gear 31s that is placed in the opposite side of the output element (ring gear 31r) with respect to carrier 31c serving as an input element to which the engine is connected. The alignment chart of FIG. 10 is obtained under a specific condition (in a motor propelled vehicle driving mode) where an engine speed Ne is xe2x80x9c0xe2x80x9d, an engine load torque Te is xe2x80x9c0xe2x80x9d, and the vehicle is propelled against a wheel load torque T0 acting on road wheels by driving second motor-generator 38 (serving as the motor operating in the motor propelled vehicle driving mode) to produce a balanced motor-generator torque Tmg20 at a forward motor-generator rotational speed Nmg20. Under the motor propelled vehicle driving mode, first motor-generator 34 (serving as the generator) is driven by a balanced torque Tmg10 acting in a direction that a reverse motor-generator rotational speed Nmg10 of first motor-generator 34 drops to xe2x80x9c0xe2x80x9d, so as to generate electricity. The generated electricity is supplied as a part of electric power used to drive second motor-generator 38 (the motor). The alignment chart of FIG. 11 shows a transition from the motor propelled vehicle driving mode to an engine start-up mode. The collinear indicated by the broken line in FIG. 11 corresponds to a lever on the alignment chart of the motor propelled vehicle driving mode shown in FIG. 10. On the other hand, the collinear indicated by the solid line in FIG. 11 corresponds to a lever on the alignment chart of the engine start-up mode in which, in order to increase engine speed Ne against the engine load torque Te, the reverse motor-generator rotational speed of first motor-generator 34 is reduced from the speed value Nmg10 to a speed value Nmg11 close to xe2x80x9c0xe2x80x9d by way of a balanced torque Tmg11 acting on the first motor-generator. At this time, there are the following drawbacks.
Regarding the alignment chart of FIG. 11, the input rotation system connected to carrier 31c includes the engine whose inertia is great. Likewise, the input rotation system connected to ring gear 31r includes the differential gear device and drive wheels and thus the input rotation system has a great inertia. The center-of-gravity G of the lever on the alignment chart indicated by the broken line in FIG. 11 is positioned between carrier 31c (or the engine) having the great inertia and ring gear 31r (or the output element) having the great inertia. For the reasons set forth above, when reducing the reverse motor-generator rotational speed of first motor-generator 34 from speed value Nmg10 to speed value Nmg11 for an engine start-up from the motor propelled vehicle driving mode, the lever on the alignment chart indicated by the broken line in FIG. 11 is rotated about the center of gravity G and changed to the lever on the alignment chart indicated by the solid line in FIG. 11. This means that a reaction force resulting from the great inertia of the input rotation system (the engine) acts to unintentionally reduce the rotational speed of the output rotation system containing the differential gear device and drive wheels, thus resulting in a temporary drop in vehicle speed. At this time, the driver may feel uncomfortable. To avoid this, a balanced motor-generator torque Tmg21 of second motor-generator 38 has to be controlled or adjusted to a greater value than the balanced motor-generator torque Tmg20 of the motor propelled vehicle driving mode shown in FIG. 10 by an incremental torque needed to prevent the undesired vehicle speed drop, when starting up the engine from the motor propelled vehicle driving mode under the same wheel load torque T0. In this case, during the motor propelled vehicle driving mode, second motor-generator 38 has to be driven by the remaining electric power obtained by subtracting an electric power corresponding to the previously-noted incremental torque from a possible battery output power to enable engine start-up. The remaining electric power means a reduced vehicle driving performance during the motor propelled vehicle driving mode. To insure the vehicle driving performance greater than a predetermined level, a large capacity of car battery must be used. In addition to the above, owing to the layout of center of gravity G put between carrier 31c (or the engine) and ring gear 31r (or the output element), there is an increased tendency for engine torque fluctuations, which may occur during the engine start-up period, to be transmitted to the output element, thus resulting in the driver""s uncomfortable feeling. To eliminate the driver""s uncomfortable feeling, for instance, it is necessary to instantaneously control a generated torque of second motor-generator 38 directly connected to the output element responsively to positive and negative engine torque fluctuations. Actually, it is very difficult to momentarily adjust or control the generated torque of second motor-generator 38, and therefore it is almost impossible to perfectly eliminate engine torque fluctuations. Such undesired torque fluctuations lead to the problem of deteriorated vehicle""s driveability.
Accordingly, it is an object of the invention to provide a hybrid automatic transmission of a hybrid electric vehicle, which avoids the aforementioned disadvantages by improving a position of a center of gravity of a lever on an alignment chart of the hybrid automatic transmission.
In order to accomplish the aforementioned and other objects of the present invention, a hybrid automatic transmission of a hybrid electric vehicle mounting thereon a prime mover, first and second motor-generators and a drive train, and capable of steplessly varying a transmission gear ratio by controlling an operating state of each of the first and second motor-generators, comprises a differential device having at least four rotating members and two degrees of freedom that determining rotating states of two members of the four rotating members enables rotating states of the other rotating members to be determined, a first member of the four rotating members serving as an input element connected to the prime mover, a second member serving as an output element connected to the drive train, a third member connected to the first motor-generator, and a fourth member connected to the second motor-generator, and a specified relationship among an inertia of a rotating system relating to the prime mover, an inertia of a rotating system relating to the output element, an inertia of a rotating system relating to the first motor-generator, an inertia of a rotating system relating to the second motor-generator, a first lever ratio of a distance between the input element and the first motor-generator to a distance between the input and output elements, and a second lever ratio of a distance between the output element and the second motor-generator to the distance between the input and output elements is determined so that a center of gravity of a lever on an alignment chart of the hybrid automatic transmission is laid out on the output element or between the output element and the second motor-generator.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.